Effects of Nanocylinders on the Whispering Gallery Modes in a Microcylinder

Optical biosensors have been studied extensively for the detection and characterization of biological entities, such as viruses, bacteria, and biomolecules. A two-dimensional (2D) microcylinder resonator (Q ∼ 2 × 10(5)) was designed, and the effects of a nanocylinder on the whispering gallery modes (WGMs) were examined numerically. For this purpose, the finite element method with COMSOL multiphysics software was employed. The perturbation of the WGM resonances can be characterized by the shift and splitting of the resonance peaks, which varies according to the position, size, and refractive index of an embedded nanocylinder. The positional dependence shows a large splitting in the region of strong electric fields, and the size dependence shows a broad peak of the splitting at R c = 110 nm . These results are attributed to the changing degree of overlap of the WGMs with the nanocylinder. The refractive index dependences of splitting show linear behavior for a nanocylinder less than 50 nm in size, and the nonlinear behavior increases with increasing size of the nanocylinder. The optical resonator system is shown to be suitable for detecting impurity particles, which are smaller than the sizes of the node and antinode regions.